DESCRIPTION (Adapted from the abstract provided by the applicant): Age is the single greatest risk factor for most neurodegenerative disorders, even those that are genetically based. This delayed onset is believed to reflect an interaction between the risk factors for a neurodegenerative disease, and the aging process itself. Oxidative damage to mitochondrial DNA accumulates in brain of older individuals in many species, including man. This observation has led to the speculation that oxidative injury to mitochondria causes loss of mitochondrial metabolic reserve during aging, and that this contributes to the age-dependent onset of neurodegenerative processes. One class of proteins uniquely situated to contribute to, or modify, these age-dependent changes in mitochondrial function are the mitochondrial uncoupling proteins (UCPs). Mitochondrial uncoupling proteins are specifically designed to impair the efficiency of energy production by mitochondria to produce heat. Outside the nervous system, UCPs regulate body weight, temperature, and the response to starvation. Recently, however, we and others have shown that these proteins also regulate mitochondrial free radical production. Three UCPs (UCP2, 4, and 5) are expressed in brain, where their function(s) is essentially unknown. Our laboratory has been studying UCP5, and has determined that it is a neuronal protein with high expression in the forebrain of both mouse and man. We also found that over-expression of UCP5 in neurons decreased mitochondrial free radical production, a potentially beneficial effect, but decreased the efficiency of mitochondrial function and enhanced the vulnerability of neurons to injury and subsequent degeneration. We hypothesize that UCP5 in brain may be a two-edged sword which trades lower mitochondrial free radical production for greater mitochondrial metabolic inefficiency. We propose to determine whether expression and/or activity of UCP5 is altered in brain during aging. We will also determine whether this results in 1) constitutively higher levels of free radical production by mitochondria in older brain, and 2) increased vulnerability of brain to metabolic stress when UCP5 expression is induced. We will first identify factors, such as hormones or caloric restriction, which regulate expression and activity of UCP5. We will then use biochemical and fluorescence imaging techniques to evaluate mitochondrial function and free radical formation. Initial experiments will be performed in cultured neurons with modified levels of UCP5 or after treatment with agents to modify UCP5 levels or activity. We will then look at how altering UCP5 expression/activity impacts mitochondrial function and free radical production in brain of old mice. For many of these experiments, we will use Thy1-YFP mice, which exhibit neuronal expression of a fluorescent protein, to allow imaging of neuronal mitochondria in brain slices. Finally, we will generate mice deficient in UCP5 to study the normal function of UCP5 in brain and to elucidate how absence of UCP5 affects mitochondrial function and radical production in brain during aging, with the eventual goal of determining whether UCP5 contributes to the age-dependence of neurodegenerative disorders.